1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor for measuring torque input into a shaft (e.g., a steering shaft), based on a variation in magnetic characteristics due to magnetostriction (or magnetic strain), and relates to an electric steering system having such a magnetostrictive torque sensor.
2. Description of Related Art
As a contactless magnetostrictive torque sensor, a magnetostrictive torque sensor for measuring torque based on a variation in magnetic characteristics due to magnetostriction is known. Such a magnetostrictive torque sensor is used for measuring a steering torque in a steering system for a vehicle (see Japanese Unexamined Patent Application, First Publication No. 2002-316658).
FIG. 6 is a diagram for explaining torque measurement using a conventional magnetostrictive torque sensor and failure (or trouble) detection for the magnetostrictive torque sensor. As shown in FIG. 6, magnetostrictive films 91 and 92 having different magnetic anisotropies are provided to a rotation shaft 99, and measurement coils 93 and 94 are respectively made to face the magnetostrictive films 91 and 92 (see Japanese Unexamined Patent Application, First Publication No. S59-164932). In the measurement principle of this magnetostrictive torque sensor 90, when a torque is applied to the rotation shaft 99, magnetic permeabilities of the magnetostrictive films 91 and 92 vary, and accordingly, inductances of the measurement coils 93 and 94 also vary. The torque is measured based on the variations in the inductances.
When such a magnetostrictive torque sensor is used, failure detection for the sensor is necessary when the torque is measured.
When using the above-described magnetostrictive torque sensor 90 having two magnetostrictive films 91 and 92, torque measurement is performed by computing a torque measurement output value VT3 based on the difference between a measurement output of one measurement coil 93 (called the first measurement output value VT1) and a measurement output of the other measurement coil 93 (called the second measurement output value VT2), and failure detection is performed by computing a failure detection output value VTF based on the sum of the first measurement output value VT1 and the second measurement output value VT2 and by comparing the failure detection output value VTF with a specific threshold.
FIG. 7 is a diagram showing output characteristics when the torque measurement output value VT3 is computed based on the following formula (1), and FIG. 8 is a diagram showing output characteristics when the failure detection output value VTF is computed based on the following formula (2).VT3=k·(VT1−VT2)+V0  (1)VTF=|VT1+VT2|−C  (2)
In the above formulas, k, V0, and C are constants.
Generally, a magnetostrictive film has temperature characteristics in which the higher the temperature, the higher the magnetic permeability. Therefore, in the magnetostrictive torque sensor 90, when the magnetic permeabilities of the magnetostrictive films 91 and 92 vary according to a variation in temperature (i.e., a temperature variation), the first measurement output value VT1 and the second measurement output value VT2 of the measurement coils 93 and 94 also vary. If the first and second measurement output values VT1 and VT2 vary as shown by the dashed lines in FIG. 7 due to a temperature variation, the torque measurement output value VT3 is scarcely affected by the temperature variation because VT3 is the difference between the first and second measurement output values VT1 and VT2. Therefore, in this case, the torque measurement output value VT3 is accurate even when there is a temperature variation.
However, the failure detection output value VTF is the sum of the first and second measurement output values VT1 and VT2. Therefore, when VT1 and VT2 vary as shown by the dashed lines in FIG. 8 due to a temperature variation, the failure detection output value VTF is also affected by the temperature variation. Accordingly, the failure detection output value VTF may exceed (or deviate from) a failure detection threshold range A (see the bold dashed line in FIG. 8), and it is determined that the torque sensor is out of order failure even when the sensor normally operates.
In addition, when the above-described magnetostrictive torque sensor is mounted in a vehicle and the magnetic field in the vehicle interior changes due to a magnet built in a road or to activation of an actuator (e.g., a starter motor) using a large current, the first measurement output value VT1 and the second measurement output value VT2 may vary. FIG. 9 is a diagram showing output characteristics for torque measurement using a conventional magnetostrictive torque sensor, so as to explain influence of variation in the magnetic field. When the first measurement output value VT1 and the second measurement output value VT2 vary as shown by the dashed lines in FIG. 9 due to a variation in the magnetic field, the torque measurement output value VT3 is scarcely affected by the variation in the magnetic field because VT3 is the difference between the first and second measurement output values VT1 and VT2. Therefore, in this case, the torque measurement output value VT3 is accurate even when there is a variation in the magnetic field.
However, the failure detection output value VTF is the sum of the first and second measurement output values VT1 and VT2. FIG. 10 is a diagram showing an example of a variation in the output value when the magnetic field varies. FIG. 11 is a diagram showing output characteristics for failure detection using a conventional magnetostrictive torque sensor, so as to explain the influence of variation in the magnetic field. When VT1 and VT2 vary as shown by the dashed lines in FIG. 11 due to a variation in the magnetic field, the failure detection output value VTF is also affected by the variation in the magnetic field. Accordingly, the failure detection output value VTF may exceed a failure detection threshold range A (see FIG. 10 and the bold dashed line in FIG. 11), and it may be determined that the torque sensor is out of order even when the sensor is operating normally.